The present invention generally relates to laboratory work enclosures or safety cabinets having an isolated work space and, more particularly, to biological safety cabinets constructed to prevent airborne contaminants within the work space from escaping from the cabinet into the ambient environment.
In general, safety cabinets have been developed for protecting a technician working with various toxic and hazardous materials, such as biological matter and radiological materials, from exposure to airborne contaminants generated during the handling of these materials. The containment or isolation of hazardous and toxic laboratory substances has generally been accomplished by providing a work area which is enclosed with a hood structure having one or more access openings to a technician. The access openings allow the technician, for example, to reach into the work area to handle the material contained in the hood structure. Since these access openings provide another avenue for transfer of hazardous and toxic airborne contaminants between the inside of the hood structure and the outside or ambient environment, it has been a well known past practice to provide a means for causing a continuous positive air flow into the hood structure through the access opening or openings. This continuous flow of air from the ambient environment through the access opening or openings prevents the escape of any airborne contaminants from the work area.
Many prior safety cabinet structures include high efficiency particulate air (HEPA) filters for filtering air being directed into the work area and air being exhausted from the cabinet into the ambient environment. The air being directed into the work area is drawn both from air recirculated from the work area and air taken in from the ambient environment through the access opening or openings. Thus, the HEPA filter which filters air entering the work area, i.e., the "supply filter", can ensure that any contaminants picked up from the work area are not recirculated back into the work area and further that contaminants from the ambient air are not circulated through the work area. The HEPA filter which filters air exhausted from the cabinet, i.e., the "exhaust filter" ensures that hazardous and toxic airborne contaminants generated by the material contained in the cabinet are not exhausted into the ambient environment. These filters must be replaced on regular intervals depending on the application.
Illustrative examples of safety cabinet structures of the above-mentioned type are found in U.S. Pat. No. 3,895,570 issued to Eagleson, Jr. on Jul. 22, 1975, U.S. Pat. No. 4,637,301 issued to Shields on Jan. 20, 1987 and U.S. Pat. No. 5,380,244 issued to Tipton on Jan. 10, 1995. Each of these patents disclose work stations or safety cabinets of the general type described above having one or more replaceable HEPA filters for cleaning air which is circulated through the cabinet structure.
In cabinets such as the one disclosed in U.S. Pat. No. 5,380,244, a transition boot is connected between the supply and exhaust plenums. Typically, these transition boots have been flexible conduits, such as vinyl conduits, which are mounted to portions of the exhaust and supply plenums. These boots must be disconnected from the exhaust plenum and/or the supply plenum to regularly replace the internal HEPA filters. Typically, the boot is permanently fastened to the supply plenum with screws that fasten one end of the boot to the supply plenum in conjunction with a metal clamping member. A typical manner of mounting the transition boot to the exhaust plenum has been to place adhesive around the boot and then apply the boot to an exhaust filter damper. After the boot is in place, a band of plastic or metal is tightened over the boot to the exhaust filter damper such that no air leaks around the connection. This type of connection is time consuming during assembly and, in addition, makes replacement of the transition boot difficult. In this regard, to remove this end of the boot during filter maintenance, the band is cut and the adhesively secured end must be peeled away. To reattach the boot, adhesive must again be applied and a new band must be secured in place all within a space that is not easily accessible.
Another problematic area in safety cabinet design is that of sizing the filters appropriately to the cabinet and controlling air flow through these filters such that a uniform air flow is created within the work area of the cabinet. It has been generally recognized that a lower pressure drop across the exhaust filter and a higher pressure drop across the supply filter will allow better control of air flow through the cabinet. In this regard, the lower pressure drop through the exhaust filter will initially allow greater air flow through the exhaust plenum than the supply plenum of the cabinet. Once this situation is created, an adjustable damper assembly may be used in the exhaust air flow to restrict the amount of air flowing through the exhaust plenum. This will correspondingly increase the amount of air being forced in an opposite direction through the supply plenum and into the work area. In the past, a larger filter area has been used in the exhaust plenum to ensure a lower pressure drop across the exhaust filter. However, this can undesirably increase the size and expense of the exhaust filter. Various manners of preventing air from being directly forced from a blower assembly through the supply filter have included diffuser vane structures, diffuser panels and blast plates mounted in the vicinity of the blower assembly outlet. None of the solutions to these solutions to the air flow control problems and the related problems of filter design have been entirely satisfactory.
It would therefore be desirable to provide an easier manner of assembling and disassembling the transition boot of a safety cabinet and a better manner of controlling air flow in a safety cabinet.